In the so-called "flameless" atomic absorption measurements, a sample to be analyzed is fed into the graphite tube atomizer of an atomic absorption spectrometer. The graphite tube is mounted between annular electrodes at the ends of the tube and a control voltage is applied to produce a heating current that heats the tube to a desired high temperature while an inert gas flow to the tube prevents burning of the graphite. When the tube is sufficiently heated, usually to a temperature above 800.degree. C., the sample in the graphite tube will be atomized to form an atomic cloud containing each of the various elements in the sample. The measuring beam of the absorption spectrometer then passes axially through the graphite tube to analyze the elements within the atomic cloud and to determine their proportional quantities by the amount of attenuation of the spectrometer beam.
Different graphite tube temperatures are required for the analysis of different samples and it is most desirable to reach the required temperature as quickly as possible so that the sample will be atomized completely as rapidly as possible. It is therefore important that a relatively high voltage be applied across the graphite tube to provide a rapid heating current and then, at the proper temperature required to atomize the contained sample, the heating current should be reduced to maintain the desired temperature. It is, of course, possible to manually switch on a higher voltage for rapid tube heating and then, as temperature approaches the desired level, manually reduce the voltage to a temperature-maintaining level. There are, however, automatic voltage adjustment systems. One said system is described in the article entitled "Temperature Controlled Heating of the Graphite Tube Atomizer in Flameless Atomic Absorption Spectrometry" by Lundgren et al, at Volume 6, No. 8, Page 1028 of "Analytical Chemistry", July 1974. That article describes a temperature-controlling circuit including a precision photodiode positioned to sense the radiation from the graphite tube surface passing through selected red filters. The output of the photodiode thus represents a particular temperature and is applied to a differential amplifier where it is compared with a reference voltage. The output of the differential amplifier controls a triac in the graphite tube primary voltage circuit so that full heating power is switched on and off as necessary to obtain and maintain a desired tube temperature. Obviously, the accuracy of this temperature control circuit depends upon the accuracy of the detector signal and aging of the detector or other variations, such as dust or other deposits on any of the optical elements, directly affect the temperature to which the graphite tube is controlled. Furthermore, the on-off control of the full heating power permits temperature over-shooting and hunting.